Investigation of the electronic properties of ultrathin films of pure bismuth and bismuth–antimony alloys is of interest, since it is found that the conductivity increases with decreasing sample thickness. The results of experimental investigation of the structure, as well as of the electrical, galvanomagnetic, and thermoelectric properties of pure bismuth and Bi_1 – xSb_x thin films (x = 0.05 and 0.12) on a mica substrate in the thickness range of 10–30 nm are presented. An increase in the conductivity with decreasing thickness of the samples, which may be induced by the presence of topologically protected surface states, is found. It is shown that the features of this effect depend significantly on the band structure of alloys. The form of the temperature dependences of the Seebeck coefficient casts doubt on the fact that the surface states have a positive effect on the thermoelectric efficiency of thin bismuth–antimony films. However, the detection of positive thermoelectric power in Bi_0.88Sb_0.12 samples may become an important factor in learning how to create the p branch of thermoelectric converters.

The increasing of charge carriers concentration in thin bismuth films

We report on the production of 200 and 600 nm thick Bi films on mica substrate with 10 nm thick Sb sublayer between Bi and mica. Two types of films have been studied: block and single crystal. Films were obtained using the thermal evaporation technique using continuous and discrete spraying. Discrete spraying allows smaller film blocks size: 2–6 $\mathsf{\mu }$ m compared to 10–30 $\mathsf{\mu }$ m, obtained by the continuous spraying. Single crystal films were made by the zone recrystallization method. Microscopic examination of Bi films with and without Sb sublayer did not reveal an essential distinction in crystal structure. A galvanomagnetic study shows that Sb sublayer results in the change of Bi films properties. Sb sublayer results in the increase of specific resistivity of block films and has no significant impact on single crystal films. For single-crystal films with Sb sublayer with a thickness of 200 nm the Hall coefficient has value 1.5 times higher than for the 600 nm thickness films at 77 K. The change of the Hall coefficient points to change of the contribution of carriers in the conductivity. This fact indicates a change in the energy band structure of the thin Bi film. The most significant impact of the Sb sublayer is on the magnetoresistance of single-crystal films at low temperatures. The increase of magnetoresistance points to the increase of mobility of the charge carriers. In case of detecting and sensing applications the increased carriers mobility can result in a faster device response time.

The reasons for increasing the charge carriers concentration in thin bismuth films are discussed. The concentration was calculated on the basis of the measured electrical and galvanomagnetic coefficients at the temperature 77K under the two-band approximation and the assumption that the charge carriers free path in the film is isotropic.